The present invention relates to semi-solid molding (SSM) of metal alloys and the equipment and methods used for SSM, and which are disclosed in many U.S. and foreign patents, for example, in U.S. Pat. Nos. 3,954,455, 4,434,837, 5,161,601 and 6,165,411. SSM is also discussed in technical publications, for example, in a book entitled Science and Technology of Semi-Solid Metal Processing, published by North American Die Casting Association in October, 2001. Chapter 4 of this publication was authored by a co-inventor of the present invention. In conventional SSM processes, it is necessary to use either a specially treated, pre-cast billet of appropriate microstructure or a slurry especially prepared from molten alloy in equipment external to a die casting press. The cost premiums associated with either the pre cast specially treated billet that must be sawed to length before using, or the slurry especially prepared in equipment external to the die casting press, have severely limited the commercial applications of the SSM processes. Also, the pre-cast billet is available from a relatively few sources, is currently made only from primary alloys, and process offal cannot be reused unless reprocessed back into a billet.
Still, SSM provides some important and highly desirable characteristics. Unlike conventional die castings, die cast parts which are produced using SSM processes can be produced substantially free of porosity, they are able to undergo high temperature thermal processing without blistering, they can be made from premium alloys, and they provide reliable high levels of strength and ductility when made using appropriate alloys and heat treatments. Because of the thixotropic nature of semi-solid slurry and the non-turbulent way that relatively viscous thixotropic slurries flow in die casting dies, the SSM process is capable of producing cast parts having thin sections, great detail and complexity and close dimensional tolerances, without the entrapped porosity and oxides which are commonplace in conventional die casting processes.